Cracking open an ancient avionics gyroscope

This artificial horizon might as well have come from an alien ship. [Mike] somehow manages to get his hands on most interesting equipment, this time its a very old piece of avionics equipment. The mechanical gyroscope functioned as the artificial horizon, and he’s going to take us inside for a look. He doesn’t spend quite as much time on it as he did that thermal imaging camera, but this electro-mechanical odyssey is just as interesting.

To get the accuracy needed to help keep a plane in the air (well to keep the pilot well-informed anyway) the device needed to be very well manufactured. [Mike] comments several times along the way on how the different rotating parts are so well-balanced and machined that they seem nearly frictionless. It appears that a lot of the positional feedback depends on wirewound resistor rings which connect to a rotating piece via a series of very fine spring wires. As the parts rotate the resistance changes and that’s what gives the feedback. There are also mercury switches to help along the way.

He does his best to explain, but to us the inner workings are still a big mystery. See if you can get a clearer picture from the video after the break.

I had occasion to take one apart myself about a month ago. I didn’t know what it was when I got it (it didn’t have any a display) but I was amazed at the precision. That they managed to balance all the internals so well is an incredible feat of engineering, and it was done before CAD.

Artificial horizons such as this one suffer from gyroscopic precession. After maneuvering the plane the attitude indicator will be slightly inaccurate and need to be tuned.

In less expensive modern analog gyros the tuning is performed by first putting the plane in a known straight and level flight configuration and then manually turning a screw on the face plate to recenter the attitude indicator. This artificial horizon had a fancy one button push mechanism to automatically recenter the attitude indicator.

Yes, good pilots religiously tune the artificial horizons after each and every maneuver, particularly when flying on instruments.

I learned about these things when I was training to fly in the airforce. The precession primarily comes from the fact that the gyroscope points to a fixed point in space, but as the Earth rotates it means the gyro points to a moving point on the Earth. They are normally linked to a compass which can be used to correct for the precession – the gyro is accurate for short timescales but tends to drift over longer timescales, whereas the compass is inaccurate over short timescales (can’t quite remember why but something to do with direction of the Earth’s magnetic field relative to the aircraft I think) but in the long run it points to the same place on Earth. As far as I remember you don’t need to recalibrate frequently – you do it on the ground as one of the start up checks and leave it at that.

You’re thinking of the Directional Indicator, which must be reset to the compass quite often due to precession. Since the artificial horizon only needs to hold the horizontal plane, it’s not as susceptible to drift. The directional indicator works a lot harder.

The artificial horizon only needs setting once per flight, unless you’re like me and suffer from a bad attitude now and then. But even then, I’ve never caged a gyro just because I wanted to watch some mammalian body dynamics. :)

I wonder how many times wings got shed in the 1920’s just because some hot shot wanted a hug?

I have a few units from that vintage and WWII. One thing to be very careful of is the use of radium on the dial markings and moving needles. It tends to flake off and you might have those flakes loose inside the unit along with dust created by the moving motor. I use a pancake Geiger counter for alpha emission, gloves and a facemask when opening these units if there is radium. Please be careful as a microscopic source of alpha radiation is particularly nasty once inhaled and enbedded in your lungs.

Tritium is a gas just like deuterium and hydrogen (both non radioactive though). It “mostly” emits alphas too but it is enclosed in an ampule with some phosphor(a bit like a fluorescent tube)to emit light. Thus the alphas are contained in the ampule as they have a low propagation range. It is this low propagation range that makes radium dangerous in a different way as a loose particle stucked in you body will keep irradiating the same spot for decades at a small range. The surrounding cells will be under the gun. So as long as the enclosed tritium container is sealed there is not much to be concerned about. If it has been broken the stuff has gone away but I would dismantle a sealed unit outside if it shows signs of damage. Hope that helps.

Hackaday, I blame you for my lack of sleep last night. First I had to watch this. Then I had to watch him reverse engineer a pill cam. Then I spent an hour watching him repair a FLIR visor. Then I spent another few hours watching Photonicinduction’s new channel. Next thing you know it’s 4am.